This invention relates to improvements in the operation and configuration of long rotary cement kilns. More particularly it is directed to a method and apparatus for enhancing the capacity and efficiency of cement clinker production in conventional wet or dry process long rotary kilns. A portion of the kiln gases is withdrawn from the rotary vessel at a point upstream, relative to kiln gas flow, of the mineral drying zone and pre-heating zone to form a kiln gas bypass stream.
In the widely used commercial process for the manufacture of cement, the steps of drying, calcining, and clinkering cement raw materials are accomplished by passing finely divided raw materials, including calcareous minerals, silica and alumina, through a heated, inclined rotary vessel or kiln. In what is known as conventional long dry or wet process kilns the entire mineral heating process is conducted in a heated rotating kiln cylinder, commonly referred to as a "rotary vessel." The rotary vessel is typically 12 to 15 feet in diameter and 400-700 feet in length and is inclined so that as the vessel is rotated, raw materials fed into the upper end of the kiln cylinder move under the influence of gravity toward the lower "fired" end where the final clinkering process takes place and where the product cement clinker is discharged for cooling and subsequent processing. Gas temperatures in the fired clinkering zone of the kiln range from about 1300.degree. to about 1600.degree. C. Kiln gas temperatures are as low as about 250.degree. to 350.degree. C. at the upper mineral receiving end of so-called wet process kilns. Somewhat higher gas temperatures exist in the upper end of long dry process kilns.
Generally, skilled practitioners consider the cement making process within the rotary kiln to occur in several stages as the raw material flows from the cooler gas exit mineral feed end to the fired/clinker exit lower end of the rotary kiln vessel. As the mineral material moves down the length of the kiln it is subjected to increasing kiln gas temperatures. Thus in the upper portion of the kiln cylinder where the kiln gas temperatures are the lowest, the in-process mineral materials first undergo a drying process and thereafter move into the hotter calcining zone and finally into the portion of the kiln where the kiln gas temperatures are the hottest, the clinkering zone adjacent to the fired lower end of the kiln cylinder. The kiln gas stream flows counter to the flow of in-process mineral materials from the clinkering zone, through the intermediate calcining zone and the mineral drying zone and out the upper gas exit end of the kiln into the kiln dust collection system. The flow of kiln gases through the kiln can be controlled to some extent by a draft induction fan positioned in the kiln gas exhaust stream.
The drying/pre-heat zone of a long process kiln is defined as that part of the kiln in which sufficient heat transfer takes place from the kiln gas stream to the mineral bed to eliminate moisture from the mineral raw material feed. In the drying zone, the mineral material is heated to a temperature of up to approximately 1500.degree. F., the temperature at which mineral carbonates begin to calcine (release carbon dioxide). To facilitate the transfer of heat in the drying/pre-heat zone, chain curtains are provided. As mineral material is introduced and travels down the length of the drying zone, the chains continually rotate between the hot kiln gas phase and the mineral material phase transferring heat from the gases to the raw materials.
One of the shortcomings of the use of conventional long kilns for cement manufacture derives from the significant quantities of dust generated in the drying zone of the kiln and carried out of the kiln in the kiln gas stream. On the average about 7-10% (but as high as about 17%) of the raw material feed on a dry basis is blown back from the drying zone as dust. The high dust loss associated with the manufacture of cement clinker in long wet or dry process kilns places long kiln operators at a significant economic disadvantage relative to cement manufacturers using the newer pre-heater/pre-calciner kilns. High dust loss not only means loss of efficiency of use of raw materials--dust recovery is limited because of contamination by volatile alkali salts--but it also requires greater capital investment in dust collection equipment and loss of energy efficiency. Every ton of raw material lost as dust consumes significant amounts of heat energy during drying. Further the long kiln operator is burden by the cost of handling/disposal of waste dust.
Another disadvantage suffered by cement manufacture in conventional long kilns relative to that in pre-heater/pre-calciner kilns derives from the internal cycling of volatile alkali salts. As the in-process mineral travels down the kiln to the hotter zones, alkali salts such as potassium and sodium sulfates in the mineral are volatilized into the kiln gas stream and carried as an alkali fume into the coolest zones of the kiln, including particularly the dust generating drying zone where the alkali fume condenses on the dust and either falls back into the in-process mineral bed or is carried out of the kiln in the kiln gas stream and collected with the kiln dust in the dust collection system. It is the presence of the significant quantities of condensed alkali salts in the kiln dust that prohibits the return of the dust to the process--adding the alkaline dust to the mineral raw material can disrupt the alkali salt recirculation equilibrium and cause unacceptably high alkalinity in the cement product.
During cement kiln operation there is significant recirculation of alkali salts from the mineral bed to the kiln gas stream and back to the mineral bed. At equilibrium conditions where the alkali salts are carried out of the kiln at the same rate at which they are introduced in fuels and raw materials, it is estimated that there is 90% trapping of the volatilized alkali. The internal cycling of high quantities of alkali salts during kiln operation constitutes a significant energy burden on the process. Large quantities of high grade heat in the hotter zones of the kiln (where the heat is needed most) are lost to alkali volatilization only to be returned to the process as low grade heat during condensation in the lower temperature zones where excess heat is not needed. The configuration of pre-heater/pre-calciner kilns allows facile installation of alkali bypass conduits through which a portion of the alkali fume-laden kiln gas stream can be withdrawn before it becomes contaminated with entrained kiln dust, thereby not only reducing the alkali load in the internal cycle, but also providing means for recovery of a product highly enriched in the volatile alkaline salts. Until now, there has been no means for establishing an alkali bypass for conventional long wet or dry process kilns.
There are many existing long kiln cement manufacturing operations, not only in the United States but in many other countries as well around the world. The development of a method/apparatus for reducing dust loss and for enhancing control of the internal alkali cycling in long kiln cement manufacturing operations will enable long kiln operators to be more economically competitive with pre-heater/precalciner kiln operators.
Thus it is one object of this invention to provide an apparatus and method to reduce the dust loss in the manufacture of cement in long wet or dry process kilns.
It is another object of this invention to reduce the alkali content of kiln dust produced during the manufacture of cement in long kilns.
Another object of this invention is to provide a method and apparatus for enhancing the capacity and economic efficiency of clinker production in conventional long wet or dry process kilns.
Another more general object of this invention is to provide an apparatus for controlling the kiln gas stream in a rotary cement kiln by allowing withdrawal of kiln gas and/or injection of air through ports in the wall of the rotary kiln vessel into the kiln gas stream at pre-determined points along the length of the rotary kiln vessel.
It is still another object of this invention to modify a conventional long wet or dry process kiln to enhance efficiency of raw material usage in cement clinker manufacture.
It is another more particular object of this invention to provide a more energy efficient method of cement manufacture in long kilns by providing a kiln gas stream bypass for long kilns.
Still another important object of this invention is to provide a method for monitoring combustion efficiency in long kilns without interference from volatilized organics from the cement raw materials.
Those and other objects of this invention are accomplished by modifying the kiln to allow withdrawal of a portion of the kiln gases from the rotary vessel at a point upstream, relative to kiln gas flow, of the mineral drying zone. By removing a portion of the kiln gas from the calcining zone, or more particularly from a point upstream, relative to kiln gas flow, of the mineral drying zone (most particularly the chain curtain section) and downstream of the hottest portions of the calcining zone, there is achieved a substantial reduction in dust generated and lost from the drying zone. Reduced dust loss derives not only from reduced kiln gas velocities in the drying zone, but also from the resultant extension of the drying zone itself. Removing a portion of the hot kiln gas stream results in an extended drying zone--the raw feed will have a greater moisture content through the chain curtain section of the kiln, effectively reducing the generation of dust in the drying zone.
Preferably the bypass is designed to withdraw at least about 10% of the hot kiln gases at a point upstream of the mineral drying/pre-heat zone. The bypass gases are cooled, for example, by mixing with ambient air and either returned to the kiln gas stream upstream of the kiln dust collection system, including for example, an electrostatic precipitator, or directed to an independent dust collection system.
A 10% reduction of kiln gas flowing through the drying zone is expected to result in an average velocity reduction in the drying zone of about 20% by the combined action of reduced mass and temperature. With that velocity reduction, the amount of dust blown by the gas stream out the upper gas exit end of the kiln will decrease significantly. The reduced dust loss allows an associated reduction in the amount of raw material for the same amount of clinker production. Of course, reduced dust loss also means less dust requiring disposal by the kiln operator and less of a dust load on the dust collection system.
It has recently been reported in the art that conventional long kilns can be modified to provide an environmentally safe and economically advantageous use of solid waste fuels. Apparatus and methods for delivering solid fuels, especially solid waste fuels, are known in the art. See, for example, U.S. Pat. Nos. 4,930,965, 4,969,407, 4,850,290, 5,078,594 and 5,083,516. The delivery of solid fuel into the calcining zone in accordance with the teachings of those patents tends to increase the temperature of the kiln gas stream, and concomitantly kiln gas stream velocity, through the drying zone with potential enhanced dust loss. The present long kiln bypass method and apparatus can be utilized in conjunction with use of the art-recognized methods and apparatus for burning combustible solids, particularly combustible waste solids, as supplemental fuel in long kilns.
According to this invention, a method is provided for enhancing the capacity and efficiency of clinker production in an operating conventional long wet or dry process kiln. The method comprises the step of withdrawing a portion of the kiln gases from the rotary vessel at a point upstream, relative to kiln gas flow, of the mineral drying/pre-heat zone and preferably downstream (relative to kiln gas flow) of the downstream end of the calcining zone or at least downstream of the hottest portion of the calcining zone. The kiln gas bypass stream is preferably recombined with the gases discharged from the exit end of the kiln at a point upstream, relative to kiln gas flow, of the kiln dust collection system. Preferably, at least 10% of the kiln gas stream is withdrawn to form the bypass stream.
The bypass stream is preferably cooled (quenched) by its mixture with ambient air optionally in combination with a water spray to precipitate alkali fume. In one preferred embodiment at least a portion of the precipitated alkali fume itself is separated from the bypass stream before the quenched bypass stream is recombined with the kiln gas stream upstream of the kiln dust collection system. Removal of at least a portion of the precipitated alkali fume from the bypass stream reduces alkali content of the dust collected in the dust collection system, even to the extent that the collected dust can be combined with mineral raw materials and returned to the cement manufacturing process without compromising clinker quality.
In a preferred embodiment the kiln bypass stream is withdrawn through a port in the rotary vessel having means for preventing in-process mineral material from flowing through said port. The port is in alignment with an annular plenum around the rotary vessel and in gas flow communication with the port. Means are provided for creating reduced pressure in the annular plenum to withdraw at least a portion of the kiln gas stream out of said port and into the annular plenum. The bypass stream can be processed to collect at least a portion of the precipitated alkali before it is recombined with the kiln gas stream.
In a preferred embodiment, the present invention provides an improvement for conventional wet or dry long rotary kilns, the improvement comprising a bypass port in the rotary vessel at a point upstream, relative to kiln gas flow, of the mineral drying zone and means for preventing in-process mineral material from flowing through said port during kiln operation. An annular plenum is provided in axial alignment with the port on the rotary vessel and in gas flow communication with the port. The plenum is coupled with a means for creating reduced pressure in the plenum to withdraw at least a portion of the kiln gas stream out the port and into the annular plenum.
Use of the present bypass apparatus of the present invention in long kilns provides multiple advantages to the cement manufacturing process. The operation of the present bypass system will allow the drying zone of the kiln to operate somewhat independently of the rest of the process. The energy demands of the calcining process are such that more than sufficient energy remains at the process boundary between the calcining and drying/pre-heating zones to accomplish the drying/preheating step. By rerouting a portion of the kiln gas around the drying/preheating zone through the bypass duct, the amount of energy supplied to the drying/pre-heating zone will more closely match the process requirements of this zone. Consequently, the volume of gases passing through this zone will be reduced, thereby resulting in lower gas velocity. The lower velocity will in turn reduce the amount of in-process material that will be entrained in the kiln gas stream and exit the kiln as cement kiln dust. The reduced velocity of the gas stream in the drying zone during operation of the bypass system results in a 40-75% reduction of kiln dust lost from the kiln, thereby resulting in increased process economy.
Pieces of chain are suspended in the drying/pre-heating zone as an internal heat exchange device. The suspended chains also trap some of the volatile salts (e.g., K.sub.2 SO.sub.4) in the process until equilibrium is reached and an internal cycle of volatile constituents is established in which these constituents leave the process at the same rate at which they are introduced in fuels and raw materials. An internal cycle of alkali salts can cause operating problems such as the buildup of material rings in the kiln. By rerouting a portion of the hot kiln gas containing the gaseous volatile constituents around the chain system, the magnitude of the internal cycle will be reduced, thus improving kiln operation. During operation of the bypass, "bypass gas" will be extracted prior to the gas entering the turbulence of the chain system. As a result, it is expected that there will be little suspended particulate matter in this bypass gas; however, the hot bypass gas will contain the alkali fume and other volatilized components--one target of the bypass arrangement.
When the bypass gas stream is recombined with the exit gas from the drying/pre-heating zone of the kiln prior to its entry into the air pollution control (dust collection) system, the thermal energy in the bypass gas ensures that the temperature of the gas is above the dew point even if the kiln exit gas itself is at or below the dew point.
Under 40 C.F.R. 266.104(g), monitoring of CO and THC in a bypass duct is allowed as a means of complying with CO and THC limits, provided that: (1) hazardous waste is fired only into the kiln and not at any location downstream from the kiln exit relative to the direction of gas flow (i.e., downstream from the bypass); and (2) the bypass diverts a minimum of 10% of the kiln off-gas into the duct. The preamble to the BIF regulation (56 F.R. 7159, Feb. 21, 1991) acknowledges that a rationale for allowing monitoring in a bypass duct is that a bypass would preclude the interference of non-fuel THC emissions from raw materials. In long kilns, the raw material is heated in the drying/pre-heating zone to a temperature sufficient to evaporate the hydrocarbons contained therein. The hydrocarbons from that heating process are contained in the kiln exit gases. The bypass duct will draw gas from the kiln prior to its entry into the drying/pre-heating zone; therefore, gases will not have been affected by the raw material heating process. Thus, the monitoring of the bypass gases is a true representation of the gases from the combustion process without interference from hydrocarbons in the raw material.
The present invention also enables enhanced clinker production capacity in long kilns. There are two limiting factors in the production of cement in long wet process kilns: (1) the kiln gas velocity in the drying zone causing dust loss; and (2) thermal loading in the sintering zone. Inherently these factors work to limit the amount of heat energy that can be delivered into the kiln for transfer for the in-process mineral. The impact of the first of those limiting factors can be minimized by use of the present bypass apparatus. Subject to the limitations imposed by factor (2) above, the bypass apparatus can be used to reduce kiln gas stream velocities in the drying zone even with significantly higher energy input into the process.
The impact of limiting factor (2) has been resolved by the recently developed technology for mid-kiln firing of solid fuels, including particularly solid waste derived fuels. The apparatus of the present invention can be used advantageously in combination with kiln modifications described in the art for allowing delivery of solid combustible materials to the calcining zone of an operating kiln as supplemental fuel for the cement manufacturing process. It is anticipated that use of the present bypass method in conventional long kilns in combination with the recently developed mid-kiln firing technology will enable up to a 40% increase in clinker production capacity, with enhanced energy efficiency and enhanced raw material conversion efficiency and without compromise of clinker quality. Indeed, the estimated 40% increase in long kiln capacity enabled by use of the present bypass with mid-kiln firing of solid fuels is limited itself only by the cooling of the main flame of the kiln by flow of the high volume of tertiary air necessary to support combustion in the mid-kiln region (in the calcining zone or at least at some point downstream of the clinkering zone). That limitation can be obviated by use of the present apparatus located at a point along the rotary vessel to add tertiary air into the kiln gas stream at a point downstream of the clinkering zone and upstream (relative to kiln gas flow) of the secondary burning zone. The use of the present apparatus for injection of air into the kiln requires, of course, that the plenum be coupled with means for creating increased pressure in the plenum relative to the recipient portion of the kiln gas stream.
It is also contemplated in accordance with this invention that the present apparatus (as a tertiary air injection device) can be located along the length of a rotary kiln at a point above the beginning of the calcining zone (where kiln gas temperature are generally 1800.degree. F. or lower) to facilitate oxidation of hydrocarbons volatilized from the mineral raw materials without detracting from process thermal efficiency.
Thus, the pressure in the annular plenum of the present apparatus can be controlled either to regulate air delivery into the kiln gas stream or to induce bypass flow depending on the function required at that point in the process to enhance kiln combustion/operation efficiency.
Additional features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiments exemplifying the best mode of carrying out the invention.